Corrosion damage is a significant threat to the safe operation of both military and commercial aircraft. Failure to detect and correctly quantify corrosion damage can lead to failure of various aircraft components. Corrosion damage is an even more significant problem with military aircraft due to the extreme environments in which they must operate.
Systems for determining damage to metal structures are known. One such system includes a thermographic system for detecting exfoliation corrosion on aircraft skins. However, this technique is limited to flat surfaces and only presents images that are visually compared with surrounding areas.
Traditional impedance plane eddy current techniques are also used to evaluate corrosion damage and material volume loss on aircraft skins. The eddy current response from a test sample is compared to the eddy current response from a reference standard. The result is a subjective evaluation of whether or not the test sample is better or worse than the reference standard. The technique is limited by the inability to produce corrosion reference standards with quantifiable defects. As a result, assessment of corrosion damage in an aircraft metallic structure has required a complete understanding of the physics involved and extensive experience of a specialist in complex and time consuming inspection techniques currently available in the industry. In other words, corrosion damage determination has been highly subjective and quality depends to a large degree upon the individual conducting the evaluation.
Eddy current inspection methods are known for detecting cracks and flaws in aircraft skin metal immediately surrounding rivets without requiring the removal of rivets or manual scanning. Such methods include positioning a probe concentrically around the rivet. In such methods, the inspection probe either provides a zero reading corresponding to no cracks or defects, or a non-zero reading corresponding to one or more cracks or defects in the surface. Thus, known systems locate only two-dimensional anomalies, such as cracks in the surface around the rivet. See U.S. Pat. No. 5,510,709.
Other known systems detect surface corrosion on metallic structures that are insulated by a coating or cover, or covered with marine growth. Such systems provide alternating magnetic fields to induce an eddy current. See U.S. Pat. No. 5,491,409.
Further, commonly assigned U.S. Pat. No. 6,285,183 discloses methods and apparatuses for determining corrosion and material volume loss at and beneath the surface to a depth of up to about 0.100 inch using eddy current principles, the entire contents of which are incorporated by reference and made a part of this application herein.
The great majority of damage to structural members due to corrosion originates from corrosion damage within the fastener holes and beneath the surface at faying surfaces. While some of the above-identified methods and devices are applicable to evaluating corrosive damage just beneath the surface at faying surfaces, a more precise system for determining deeper sub-surface corrosion and/or material volume loss is desirable.